Hydrophilic colloids produced by various species of Xanthomonas bacteria are exocellular heteropolysaccharides containing mannose, glucose, glucuronic acid, o-acetyl mannose groups, and pyruvic acid residuces ketalically linked to certain of the terminal D-mannose residues in the polysaccharide molecule.
Xanthan gum, also referred to as xanthan and as polysaccharide B-1459, is an exocellular heteropolysaccharide which is produced by Xanthomonas campestris.
The exocellular heteropolysaccharides are produced by certain species of Xanthomonas bacteria when these bacteria are grown aerobically in aqueous nutrient solutions. Such nutrient solutions contain, in addition to the usual growth promoting ingredients, a water soluble carbohydrate as a carbon source that can be readily assimilated by the bacteria, e.g. glucose.
The Xanthomonas heteropolysaccharides exhibit marked thickening properties as well as thixotropy in aqueous solutions, and are used extensively in various industries. Due to their non-toxic properties they have many applications in the food industry, as well as other industrial applications. Recently, they have been used for the secondary and tertiary recovery of oil deposits.
Processes for the preparation of such heteropolysaccharides are described in the following patents:
______________________________________ Document No. Date Title ______________________________________ U.S. Pat. No. Sept. 19, 1961 Method of Producing 3,000,790 an Atypically Salt- Responsive Alkali- Deacetylated Poly- saccharide. U.S. Pat. No. Feb. 6, 1962 Process for Synthesiz- 3,020,206 ing Polysaccharides. U.S. Pat. No. May 17, 1966 Fermentation Process for 3,251,749 Preparing Polysaccharides. U.S. Pat. No. Sept. 6, 1966 Biochemical Synthesis of 3,271,267 Industrial Gums. U.S. Pat. No. Oct. 25, 1966 Fermentation Process for 3,281,329 Producing a Heteropoly- saccharide. U.S. Pat. No. July 2, 1968 Process for Producing a 3,391,060 Polysaccharide. U.S. Pat. No. July 2, 1968 Process for Producing 3,391,061 Polysaccharides. U.S. Pat. No. Feb. 11, 1969 Process for Preparing 3,427,226 Polysaccharides. U.S. Pat. No. March 18, 1969 Process for Producing a 3,433,708 Polysaccharide. U.S. Pat. No. July 15, 1969 Process for the Production 3,455,786 of Polysaccharide Gum Polymers U.S. Pat. No. Feb. 23, 1971 Nitrogen Source for Im- 3,565,763 proved Production of Microbial Polysaccharides. U.S. Pat. No. July 20, 1971 Processes for Carrying Out 3,594,280 Polysaccharide-Producing Fermentations. U.S. Pat. No. Oct. 10, 1978 Process for Producing 4,119,546 Xanthomonas Hydrophilic Colloid, Product Result- ing Therefrom, and Use Thereof as Displacement of Oil from Partially De- pleted Reserves. German July 3, 1980 Verbessertes Fermentations- DE 29 47 740 verfahren zur Herstellung von Xanthan. ______________________________________
However, prior art processes for the large scale production of Xanthomonas heteropolysaccharides suffer from serious drawbacks due to the unique properties of these products. The heteropolysaccharides produced by the Xanthomonas culture in the fermentation medium thicken the medium itself, causing problems in oxygen transport and even in oxygen distribution in the fermentation medium. Such problems can arise even with low yields of the heteropolysaccharides.
It had been found that the application of large shear forces during fermentation suppresses the thixotropic properties of the heteropolysaccharides for a short period of time. Accordingly, the fermentation is carried out in reaction vessels, with vigorous stirring, using a flat bladed, multi-turbine mixer. When considerable stirring energy is used, it is possible to keep the reaction product, which otherwise would quickly gel, in a fluid state for a short while, thereby allowing adequate oxygen transport. In the literature, there are indications that a maximum yield of about 5 wt. % or even higher, calculated as dry substance, and based on the fermentation make-up, can be obtained. In practice, such yields are never obtained. A yield of 2-3 wt. % xanthan (dried substance) is regarded as a good yield, and yields of this magnitude will only be possible when using the above described conditions, i.e. by the use of large amounts of energy.